An interactive geological map of the British Isles and the world, coloured in the manner of the 1969 Geological Map of the British Islands. What lies under your town, how old is it, and why do England's layers tilt down towards London?
In 1969 the Geological Survey published the fifth edition of its Geological Map of the British Islands: one sheet, every rock in two islands, and a key of about forty colours that generations of students learnt by heart. Chalk a soft grey green, the Coal Measures grey, the Carboniferous Limestone sky blue, the Keuper red, the old Highland schists in pale pinks and yellows. I have borrowed those colours, and only the colours, and put them on today's open digital maps.
The heart of the page is Britain and Ireland: 315,859 square kilometres of bedrock from the British Geological Survey's 1:625,000 map and Geological Survey Ireland's 1:500,000 map, every unit clickable, with the ice-age deposits that hide much of it on a layer of their own. Around it lies the world, from Macrostrat's compilation of regional maps, in 4,639 map units. Switch to the international period colours, to rock types or to a smooth scale of age, drag the time slider, or press play and watch the islands build themselves, oldest rock first.
A word on what you are looking at. Bedrock is the solid rock under the soil and under the loose sands, gravels and clays the ice ages left behind. Each colour is a map unit: a body of rock the surveyors grouped by age and type. Ages are the range the unit spans, as the source gives it, so a unit "354 to 290 million years old" may hold rocks from anywhere in that span. At this scale the maps are generalised: lines are drawn to about a fifth of a kilometre in Britain, and to several kilometres in the world view.
Walk from the Lake District to London and you walk forwards through time. On the line from the summit of Scafell Pike to Charing Cross the first rock is Caradoc Rocks (Undifferentiated), about 458 million years old; the last is the Thames Group, the London Clay and its neighbours, about 55 million years old. In between come the Carboniferous rocks of Lancashire, the red Triassic of Cheshire and the Midlands, the Jurassic limestones of the Cotswold edge, and the Chalk of the Chilterns.
The reason is that the layers of England, laid down one on top of another, have been tilted gently down to the south east and planed off. Their edges come to the surface in bands, each younger than the one to its north west. Across all of England and Wales the rank correlation between how far south east a point lies and the age of the rock beneath it is −0.76 (−0.78 for England's sedimentary rocks alone). Wales, the Lake District and the Highlands break the pattern with older, harder, crumpled rocks, which is why they are the high ground.
The Carboniferous covers more of Great Britain than any other period: 17.5% of the land, from the limestone dales of the Pennines to the coalfields. It covers 54.7% of the Republic of Ireland, where the pale blue of the Carboniferous Limestone floods the middle of the country on the 1969 map. In Scotland the largest share is Neoproterozoic (40.8%), the Moine and Dalradian schists of the Highlands; in Wales it is the Silurian (34.7%); in England the Carboniferous (21.6%) just leads the Cretaceous (21.3%).
By rock type, mudstones, siltstones and shales cover 28.9% of Great Britain, sandstones 20.9% and metamorphic rocks 15.0%. Where people live is another matter: of the people in Great Britain's towns of 15,000 or more, 33.8% live on Palaeogene rocks (mostly London) and 24.4% on Carboniferous ones.
The oldest rocks in the British Isles are the Lewisian gneisses of the Outer Hebrides and the far north west of Scotland. The BGS map dates them only as "Archaean to Proterozoic"; the oldest of them, in the Outer Hebrides, formed as igneous rocks between about 3,200 and 2,800 million years ago and were later crushed and reheated deep in the crust. They cover 4,299 km² on this map, 1.86% of Great Britain, and 54.1% of that lies in the Outer Hebrides.
The youngest bedrock is in Norfolk and Suffolk: the shelly sands and gravels the BGS maps as Neogene to Quaternary Rocks (Undifferentiated), 3,602 km², laid down in the last few million years. The youngest volcanic rocks are the Palaeogene lavas and intrusions of Skye, Mull, Rum, Arran and Antrim, around 60 million years old, which cover 6,417 km² of the British Isles, more of it in Northern Ireland (3,668 km²) than in Great Britain (2,660 km²).
Chalk, the bright white limestone built from the shells of microscopic algae in a Cretaceous sea, covers 19,238 km² of Great Britain (8.3%) and 14.7% of England. On the map every one of the famous white cliffs I checked falls on the White Chalk Subgroup: Dover, Beachy Head, the Seven Sisters, Old Harry, Durdle Door and Flamborough Head. In Northern Ireland the Chalk survives under the Antrim lavas and peeps out in a thin white rim (154 km²).
The Coal Measures, the grey of the 1969 map, cover 11,828 km², 5.1% of Great Britain. Yet 264 of the 1,426 cities and towns in OS Open Names (18.5%) stand on them, 3.6 times what their area alone would give, and so do 18.2% of the people in Britain's towns of 15,000 or more. The industrial towns grew where the coal was: Sheffield, Leeds, Bradford, Swansea, Newcastle, Wolverhampton, Stoke-on-Trent, Wigan.
Most of what lies at the surface is not bedrock at all. The ice sheets and their meltwater, the rivers and the bogs, have spread a cover of loose deposits over 54.0% of Great Britain: 65.1% of Scotland, 50.5% of England and 34.4% of Wales. Till, the stony clay dumped by glaciers and once called boulder clay, covers 29.3% on its own. Turn on "Ice-age deposits" in the map layers to lay them over the bedrock.
Switch the map to "The world" and the same colours spread over every continent. On the Macrostrat map 23.8% of the dated, ice-free land is Precambrian, 16.2% Palaeozoic, 21.7% Mesozoic and 38.3% Cenozoic. The old rock gathers in shields, the worn-down cores of the continents: the Baltic Shield of Scandinavia and Finland, the Canadian Shield around Hudson Bay, and the old cratons of Africa, Brazil, India and Western Australia.
The youngest volcanic rocks, from the last 2.6 million years, cover 824,618 km² of the map (0.62% of the dated land). The biggest areas are in the United States, the lava fields of Saudi Arabia, Ethiopia and Kenya along the East African Rift, and Mexico; Iceland, the size of Ireland, has 31,313 km².
Click a rock on the map, or search a place, then press "Where else in the world?" on its card. The world map lights up every unit whose age range overlaps it.
The colours below were sampled from the index swatches of the 1969 map and keyed to today's BGS and GSI units by group and age. Where the 1969 map named a rock group (Chalk, Lias, Oolite, Wealden, Coal Measures, Millstone Grit, Old Red Sandstone, Moine, Dalradian, Lewisian), the matching modern units take its colour; elsewhere a unit takes the colour of its period, or of its kind of igneous rock.
Britain and Ireland. I took every bedrock polygon of the British Geological Survey's Geology 625k (DiGMapGB-625) map and of Geological Survey Ireland's 1:500,000 bedrock map, gave each map unit an age range in millions of years, a period, a rock type and a 1969 colour class, and drew them onto a grid in the web map's own projection (zoom 9, about 175 m on the ground). BGS covers Great Britain, the Isle of Man and Northern Ireland; GSI is used for the Republic of Ireland. BGS ages come from the map's own fields, on an older time scale (the Carboniferous begins at 354 million years rather than today's 359); GSI ages come from its age brackets with today's period boundaries. Where BGS gives only "Archaean" as the oldest age, I use 3,200 million years, the oldest protolith age of the Lewisian gneisses reported by BGS. Superficial deposits come from the same BGS map.
The world. Macrostrat's generalised "carto" vector tiles at zoom 4, drawn at about 4.9 km per pixel at the equator, kept to Natural Earth's land. Macrostrat stitches together regional maps of very different detail: the geological maps of North America, Europe (1:5 million), Africa (1:10 million), Australia (1:2.5 million) and the Arctic, and a generalised geology of the world for the rest (sources below). When you zoom in on the world view, the page asks Macrostrat for its live, more detailed tiles if your browser can reach them.
Measuring. Areas are counted on the grids with each pixel weighted by its true area. A unit's period is the period containing the middle of its age range (Precambrian units older than 1,000 million years by their oldest age). The time slider uses the oldest age of each unit for "older than" (the rock had begun to form by then) and the youngest for "younger than". Towns are placed by their OS Open Names or GeoNames point.
What loads live. The geology, hill shading, rivers, lakes, sea, place names, faults and charts are all bundled with the page. The base maps (Esri and OpenStreetMap), the Esri labels and relief tiles, the fonts and the Leaflet map library load from the internet. In the world view, Macrostrat's detailed live tiles (drawn with the Leaflet.VectorGrid library) and the extra detail on a clicked spot come from Macrostrat's servers when they can be reached; if not, the page says so and keeps the bundled map.
Limits. These are small-scale maps: lines are generalised, small outcrops are merged, and the world map mixes sources of different detail and vintage, with seams where they meet. A unit's age is its stratigraphic range, not the age of every rock in it. The rock types are my own grouping of the surveys' descriptions. The colours are a homage to the 1969 map and its index, not a reproduction of the map, and some of today's units do not fit its classes neatly.